The invention relates to the correct positioning of successive originals in a photographic strip at a copying station, a severing station, or the like, based upon the optical detection of reference marks, usually notches associated with the successive originals.
When a photographic strip is transported through a copying station, original by original, it is important that strip transport be stopped each time with the original correctly positioned. This is likewise true for stopping transport of such strip at a severing station, at which a cut is to be made exactly midway between successive originals. To establish correct positioning, such strips are conventionally provided with edge notches, one per original, and very accurately located relative to the leading and trailing ends of the respective originals. Such notches are for example provided by first running the strip through an automatic notcher, and the notches are typically of semicircular or triangular shape. In particular it is the centerlines of the notches which are spatially correlated with the respective originals. The centerlines are lines extending perpendicular to the longitudinal direction of the strip and symmetrically bisecting the notches.
Such automatic markers are very reliable with respect to establishment of such centerlines; i.e., the centerlines are typically perfectly placed and not upstream or downstream of the correct location. In contrast, the depth to which the notch extends from the edge of the strip towards the longitudinal centerline of the strip typically exhibits a certain amount of variation. This is traceable to problems of guiding the strip through the automatic notcher. The strip sometimes moves in transverse direction small distances towards and away from the actual notching element. When the notches are semicircular, triangular or otherwise not rectangular, this variation in penetration depth results in corresponding variation in the length of the notch measured in the direction of strip elongation at the notched edge of the strip. Conventional notch-referenced positioning systems of the type in question are referenced to the leading ends of the notches. However, because of the variation in the notch length, there is corresponding variation in the distance between the leading end of a notch and its centerline. As a result, there is corresponding variation in the positioning of successive originals when strip transport is interrupted, e.g., the positioning of successive originals at the copying or severing station.